Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. In a typical optical communication system, switching systems located along the fiber spans connect the spans of optical fibers. These switching systems are used both to route the optical signals to their destination, and to add and remove optical signals from the optical fibers.
Generally, the two primary communication-switching technologies in use today are circuit switching technology and packet switching technology. Currently, optical switching systems are primarily used in circuit switching applications in which an end to end communication path is established for a relatively long period of time. In circuit switching applications, switching speeds, or changes in state of the optical switching element, on the order of 10 milliseconds (ms) are typical.
Packet switching technology, on the other hand, divides information into packets where each packet is switched individually, and may traverse the communication network using a different path. Typically, packet switches have been implemented in the electrical domain to switch electrical signals because, due to the nature of the packetized data, packet buffering is often required to prevent two packets from being transmitted simultaneously to the same output of the switch. Due to the difficulty in buffering an optical signal, optical packet switches have not been commercially developed. However, it may be desirable to include an optical packet switching engine as part of a larger electrical packet switch. Furthermore, because each communication packet typically has a duration of less than 1 ms, conventional optical switching systems that are employed in circuit switching technologies are incapable of changing state quickly enough to be used in packet switching applications.
Previous attempts at fabricating an optical switch engine that can change state sufficiently fast to be useful as a packet switch have resulted in optical switches that are large, inefficient and that allow an unacceptably large amount of cross-talk between the switched signals. One such optical switch uses a substrate of lithium niobate (LiNbO3) upon which are fabricated a pair of coupled optical waveguides and a means of altering the refractive index of one or both waveguides.
Another attempt at an optical switching arrangement suitable for use as a packet switch uses a matrix of intersecting input and output waveguides. Each intersection is arranged in a “Y” configuration such that an optical input signal is divided into two signals, where each signal is approximately ½ as intense as the original input signal. An amplifier is located in each arm of the “Y.” Forward biasing one of the amplifiers while reverse biasing the other amplifier performs the switching function, allowing light to continue through the intersection or be switched to the intersecting waveguide.
A drawback of such an optical switch is that while the forward biased amplifier amplifies the light, the amplification lowers the signal-to-noise ratio of the optical signal. For a few elements, this drawback is relatively negligible. However, when multiplied over many switch elements, this drawback can become significant.
In the past, this switching technology has been implemented as a “matrix” switch. In a matrix switch, a plurality of input waveguides intersects a plurality of output waveguides, thus forming what are referred to as “crosspoints” at the intersections. One of these switches, including the “Y” splitter and two amplifiers, is located at each crosspoint. In such an arrangement, the maximum number of crosspoints that an optical signal may traverse is 2N−1, where N is the number of inputs and outputs. The maximum signal-to-noise ratio degradation (for optical signals traversing many crosspoints) in large matrix switches limits their use in a packet switching network. Further, depending on the positions of the inputs and outputs, one input signal will traverse a different number of crosspoints than another input signal. Signals taking shorter paths through the switch will have a better signal-to-noise ratio than signals taking a longer path and will experience varying amounts of cross-talk due to the different path lengths through the matrix switch.
Therefore, there is a need in the industry for an optical switch capable of fast changes of state, having low cross-talk and balanced signal-to-noise ratio performance, and capable of being economically manufactured to be useful in a packet switching environment.